Five thousand years ago, a comet whizzed through space, shining so brightly that the builders of Stonehenge must have wondered about its beauty and splendor. This mountain-sized chunk of frozen dust, gas, and ice began its journey at the edge of the solar system. If we follow its path, as it gets closer to the Sun, it begins to change, forming a bright tail of vapors. The impressive tail in reality is only the thinnest veil of material, nearly a vacuum. The comet passes by Mercury, the closest planet to the Sun. Mercury is a cratered world half the size of Earth.
Moving outward from the Sun, the comet passes by Venus. The second planet from the Sun is hidden below a thick, poisonous atmosphere that traps heat, bringing the surface temperature to 900 degrees Fahrenheit through the day and night.
In 1997, it passes by Earth and its moon, when it is viewed and named for amateur astronomers Alan Hale and Thomas Bopp. As the comet passes by, our planet blocks the light of the Sun, causing the Moon's surface to dim and redden in an eclipse.
Beyond Earth, the comet encounters Mars, a world 140 million miles from the Sun with a thin atmosphere and a frozen, desert surface. In the future, colonizers from Earth may be able to gaze on other comets in the sky over the red planet.
Jupiter's intense gravity modifies the orbit of the comet, as it did the comet Shoemaker-Levy 9, which crashed into the giant planet in 1994. A smaller planet would be devastated, but Jupiter escapes with only a sooty spot that lingers briefly before disappearing. Jupiter is circled by Io, Europa, Ganymede, and Callisto -- each a unique world.
At 800 million miles from the Sun, the comet passes Saturn, another giant planet with swirling clouds and storms. Around Saturn is a half-mile-thick system of rings made of car-sized chunks of ice. Saturn's largest moon, Titan, is home to seas of liquid methane.
Farther from the Sun is Neptune, another gas giant planet, and its moon Triton. Triton is large enough to be a planet. The frozen moon has geysers of frozen nitrogen that erupt twelve miles above the surface, where they bend 90 degrees and form a trail over 100 miles long.
Passing by Pluto and Charon, dwarf planets at the edge of our solar system, the comet is in darkness; the Sun appears as little more than a bright star.
The comet we have followed formed with the Sun and planets from a cloud of dust and gas. As the Sun began to shine five billion years ago, the comets were expelled from the inner solar system, forming a swarm of icy objects that extends halfway to the nearest star. With a small nudge, one of these objects can fall toward the Sun to become a blazing comet. Most, however, will remain drifting at the edge of the solar system, never to streak across our sky.